In hybrid vehicles or motor vehicles having electric drive or fuel-cell drive systems, as a rule voltages are used that can be more than 100 volts. Voltages greater than 60 volts are referred to as “high voltage.” For reasons of personal safety, high-voltage on-board electrical systems in motor vehicles must be capable of being switched off, and must be capable of being discharged within a specified time span. For this purpose, conventional high-voltage on-board electrical systems include active and/or passive discharging devices. The high-voltage power supply system in a hybrid vehicle or in a motor vehicle having electric or fuel-cell drive—often also referred to as a DC voltage intermediate circuit—includes in the simplest case an energy source in the form of a battery, a voltage converter having a DC voltage intermediate circuit capacitor that acts as a buffer capacitor for stabilizing the operating voltage of electrical consumers, one or more electric machines, and further high-voltage consumers. After separation of the high-voltage power supply from the energy source, which can take place for example by pulling out the corresponding plug connector during maintenance work, or may also be caused by an accident, all energy stores connected to the high-voltage system or to the DC voltage intermediate circuit must be discharged. Here, capacitors, such as an intermediate circuit capacitor, or for example also motors being turned off, can act as energy stores. A standard passive discharge device, i.e., a discharge device whose function is ensured even in the absence of all supply and control lines, provides in the simplest case an ohmic discharge resistance permanently connected in parallel to the intermediate circuit capacitor.
German Patent Application No. DE 10 2004 057 693 A1 describes a device for the rapid discharging of a capacitor, in particular an intermediate circuit capacitor, the capacitor being connected to an electric machine via an inverter, and being connected via a DC converter to a further electric charge store, in particular a battery. Here, the DC converter includes devices that bring about a rapid discharge of the capacitor when there is a corresponding controlling of the DC converter. In particular, at the battery side a resistor is connected by a switch to the DC converter in order to take over the electric power that is to be removed.
German Patent Application No. DE 10 2007 047 713 A1 describes a method for discharging a high-voltage system, in particular a DC voltage intermediate circuit, capable of being connected to an energy source via at least one resistor, in which the at least one resistor is used as a common resistor both for the charging, or pre-charging, process and for the discharge process of the intermediate circuit capacitors of the DC voltage intermediate circuit.
A further device, described in German Patent Application No. DE 10 2008 010 978 A1, for discharging an electrical system or an electric component, including a switchable resistor, provides that the switchable resistor includes a PTC resistor and a switch, which are thermally coupled, and that the control terminal of the switch is connected to the system voltage.